The present invention relates to a developing agent used in an image forming apparatus such as an electrostatic recording apparatus and an electro-photographic apparatus, particularly, to a yellow developing agent used for forming a full color image.
The binder resin used in general in a developing agent includes, for example, a thermoplastic resins such as polystyrene, styrene-acryl copolymer, styrene-butadiene copolymer, polyester resin, polyester-styrene-acryl hybrid resin, epoxy resin, and polyether polyol resin. These binder resins are designed such that the melt viscosity is lowered as required by the heating in the fixing step, an elasticity is maintained under high temperatures in order to prevent the offset, and that the binder resins have a glass transition point higher than the storage temperature so as to prevent a lump of the toner from being generated during the storage.
However, since each of the thermoplastic resins has a polar group, it is possible for the thermoplastic resin to have a slight absorption in near ultraviolet wavelength region. Also, since a partial crystal is present in the resin, the light transmitted through the resin is scattered, with the result that the light transmittance of the resin itself is not sufficiently high. What should also be noted is that the ratios and the molecular weight distribution of the monomers are adjusted for adjusting the thermal characteristics. However, the degree of freedom of the design for satisfying each of these characteristics is not necessarily high.
The color developing agent used for forming a full color image is required to have a sufficient transparency in order to permit the color development to be satisfactory when the developing agents of various colors are superposed one upon the other. Also, as a means for improving the transparency, it is necessary to improve the transparency and the dispersion capability of the materials themselves. At the same time, it is required to obtain a uniform fixed surface low in roughness in order to suppress the reflection from the surface of the formed image.
Under the circumstances, it is unsuitable to use resin that is colored or poor in transparency under the influence of the crystallinity as the binder resin of the color developing agent. The melting characteristics of the resin are also important in selecting the binder resin of the color developing agent.
Such being the situation, it was customary to use, for example, a polyester resin having a low molecular weight, which tends to be subjected to a sharp melt, and low in degree of coloring as the binder resin for a color developing agent. However, the polyester resin is not sufficiently satisfactory in terms of the transparency. Also, the polyester resin has many functional groups and, thus, is not sufficiently satisfactory in the environmental stability, and gives rise to the problem in terms of the life of the color developing agent that the charging amount tends to be fluctuated. In addition, the polyester resin is low in its elasticity in the melting step so as to give rise to the problem that the offset tends to be generated.
As a measure against the offset, generally employed is a method of coating the fixing roller with an oil. However, the oil coating method gives rise to the problems that the machine used is rendered bulky, that it is necessary to replenish the oil periodically, and that the formed image is stained with the oil and is rendered rough. Under the circumstances, various measures are taken not to use an oil.
For example, it is possible to improve the resistance to the offset properties by using a resin having a high elasticity. In this case, however, the image is rendered mat tone so as to impair the color development and the permeability through an OHP. Therefore, in order to maintain the glossiness and the transparency, proposed is a method of, for example, dispersing a wax in a sharp melt resin.
A wax having a low melting point is used in order to permit the wax to be melted even under a low fixing temperature so as to exhibit mold release characteristics. Known waxes meeting the particular requirement include, for example, natural waxes such as carnauba wax and rice wax as well as fatty acid ester wax, fatty acid amide and paraffin wax. These waxes certainly have a melting point lower than that of PP, PE waxes widely used in a monochromatic toner, but tend to give rise to a poor dispersion to be rendered poor. These waxes give rise to the problems that, if the wax fails to be dispersed uniformly, the transparency of the toner is lowered so as to render the color tone poor and, if the dispersion is excessively fine, it is impossible to obtain a sufficient resistance to the offset problems. Also, if a wax is simply dispersed in the resin of the ordinary gloss specification, it was difficult to obtain a sufficient resistance to offset. Further, since the wax has a low melting point, problems tend to be generated if a wax is added to the resin. For example, the storage characteristics and the life characteristics of the developing agent tend to be rendered poor.
In order to improve the transparency of the material itself, it is important for the pigment to be dispersed uniformly in the binder. In order to prevent the pigment particles from being agglomerated within the developing agent, employed can be a master batch method, in which a master batch is prepared in advance by dispersing large amount of pigment particles in a small amount of the binder resin, followed by adding the master batch thus prepared to the binder resin.
The master batch method noted above is certainly effective. It should be noted in this connection that the chemical affinity between the binder resin and the pigment is related basically to the dispersion capability of the pigment particles in the developing agent. Particularly, yellow is a base color for forming various colors, and a yellow developing agent is required to exhibit a sufficient transparency. However, the yellow pigment in particular fails to exhibit a sufficiently high chemical affinity with the general-purpose binder resin, resulting in failure to obtain a sufficient dispersion.
Benzidine yellow, which is a generally used yellow pigment, is one of diazo series metal free pigments.
Benzidine yellow is satisfactory in transparency and coloring power and is relatively satisfactory in its chemical affinity with the binder resin. However, the decomposed material of benzidine yellow is harmful and is designated as a cancinogenic substance in the Blue Angel Mark, which is a safety standard in Germany. Therefore, it is undesirable to use benzidine yellow in view of the environmental contamination. Such being the situation, it is desirable to develop a yellow developing agent utilizing a safe yellow pigment replacing benzidine yellow.
An object of the present invention is to obtain a yellow developing agent, which is safe in view of the environmental contamination, has a high transparency, is satisfactory in the color reproducibility and the resistance to the offset problem, and exhibits a high stability in the resistance to the environmental problem and the life.
Another object of the present invention is to provide a color image forming apparatus capable of forming a high quality color image by using a yellow developing agent, which is safe in view of the environmental contamination, has a high transparency, is satisfactory in the color reproducibility and the resistance to the offset problem, and exhibits a high stability in the resistance to the environmental problem and the life.
According to a first aspect of the present invention, there is provided a yellow developing agent, comprising a binder resin containing mainly a copolymer resin between a norbornene derivative and an ethylene derivative, a natural wax, and a yellow coloring material containing mainly benzimidazolone pigment.
According to a second aspect of the present invention, there is provided a full color image forming apparatus, comprising at least one image carrier, a plurality of developing devices arranged to face the image carriers, and a fixing device, wherein one of the plural developing devices houses a yellow developing agent, comprising a binder resin containing mainly a copolymer resin between a norbornene derivative and an ethylene derivative, a natural wax, and a yellow coloring material containing mainly benzimidazolone pigment.
In the present invention, a copolymer resin of a norbornene derivative and an ethylene derivative, the copolymer resin being transparent and low in polarity, is used as the binder resin. A natural wax is dispersed in the binder resin, and a benzimidazolone pigment is used as a yellow pigment that is safe in terms of the environmental problem so as to obtain a yellow developing agent having the wax dispersed therein satisfactorily, excellent in transparency, and satisfactory in the environmental characteristics and the life characteristics.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the present invention. The objects and advantages of the present invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.